Jump to content

Chemical and Process Engineering Resources

Distillation Pilot Plant Design, Operating Parameters and Scale-up Considerations

Dec 13 2010 11:53 AM | Chris Haslego in Separation Technology *****

Tower Diameter to Packing Size Ratio

Most of the early laboratory distillation data were taken in small columns, say 150 mm (6 in.) or less. Only very small random packings, viz., 3 to 12 mm (0.12- 0.5 in.) in size could be tested in such columns. There were several reasons for this. As the diameter of the pilot plant distillation column increases, in addition to the increase in installed cost, the cost of operating utilities, viz., reboiler steam and condenser cooling water increase proportional to the square of the tower diameter. Thus there is strong economic incentive for keeping the tower diameter as small as possible without affecting the quality of the test data.

One consideration was the rule of thumb that the test tower diameter should be at least 10 times the size of the packing. The rationale behind this rule is that if larger packings were used, the wall area surrounding the packed bed would be a significant fraction of the packing area, and as such the column wall would provide a significant portion of the mass transfer area. It follows, based on this reasoning that when scaling up such data to large towers some derating would be necessary.

On the other hand, it can be argued that in a small tower, the gap between a bed of large packings and the tower wall can cause partial short-circuiting of liquid and vapor through these gaps. For structured packings, wall wipers minimize this problem. In large commercial towers the effect of such gaps will have negligible effect on packed bed hydraulics.

Most of the commercial size random packings fall in the size range of 15 mm to 90 mm (0.6-3.5 in.), and the structured packings has crimp heights in the range of 6 mm to 30 mm (0.25-l .2 in). But the majority of random packings sold commercially fall in the size range of 25 to 70 mm (1 - 2.8 in.), while the majority of commercially sold structured packings have crimp heights in the range of 8 mm to 12 mm (0.3-0.8 in.). With the 387 mm I.D. pilot distillation columns that NCPPC operates, it was found possible to test random packings in the size range of 15 mm to 70 mm (0.6-2.8 in.); the column I.D. to packing size ratio ranged from 26 to 5.5. In the case of the structured packings that were tested in these towers, the column I.D. to crimp height ratio ranged from 13 to 65. Based on experience with commercial installations, the test data taken in a 387 mm (15.25 in.) I.D. column gives reliable design data for commercial size columns. As mentioned earlier, the FRI columns have relatively large diameters, viz., 1213 mm (47.75 in.) and 2438 mm (96in.), probably because they were originally designed for testing trays. But the SRP columns which were built in 1986 have 429 mm (16.875 in.) I.D., because they were designed primarily for testing packings. Similarly another distillation pilot column operated by the Delft University of Technology in the Netherlands has an I.D. of 450 mm (17.72 in.) (Olujic et al., 1992). Thus, a distillation pilot column of approximately 400 mm (16 in.) I.D. gives reliable test data on random and structured packings. This type of test data along with reliable distribution technology can be used, without any scale-up factor, to design commercial distillation columns.

Distribution Technology

Factors to be considered in selecting liquid distributors for a distillation test tower are:

  • Turndown ratio and height of the distributor
  • The number and size of distribution points (orifices) per unit tower cross-sectional area
  • The liquid flow variation allowed between distribution points
  • The layout of the liquid distributor points over the tower cross-sectional area

It is common practice, when testing a packing, to cover the complete operating range of the packing. In the authors' experience, the typical turndown ratio is 5:l. And, it is not uncommon to have a 7:l turndown ratio. Several types of liquid distributors are used for distillation tests. Except for the notched weir-trough distributor (which happens to have high turn-down ratio), spray distributor (which is seldom used in distillation), most of the distillation distributors fall into one of the following three categories.

  • Orifice-pipe arm distributors
  • Orifice-pan distributors
  • Orifice-trough distributors

Let us first consider the design of orifice-plate and orifice-trough distributors. Both of these types of distributors are open at the top. In the orifice pan distributor, the gas flows through specially designed risers as well as the area between the pan and the tower wall. The rest of the pan area is available for locating liquid orifices. In the orifice-trough distributor, the liquid is held in specially designed troughs with liquid orifices at the bottom and/or on the sides of the troughs; the rest of the tower cross-sectional is available for gas flow.

For a given orifice size, the flow rate through the orifice is approximately proportional to the square root of the liquid head, when the orifice is running full of liquid. Therefore, for a given set of orifices at a fixed elevation, the required head of liquid above the orifices is proportional to the square of the liquid flow rate. Thus a 2: 1 turndown ratio in flow requires a 4: 1 ratio of liquid head. Typically the minimum liquid head required for predictable flow of liquid through the orifice is about 50 mm (2 in.). Thus the liquid head required at maximum flow rate for 2: 1 turndown is 200 mm (8 in.). For 5: 1 turndown the maximum required is 1250 mm (50 in.), and for 7: 1 turndown the maximum head required is 2450 mm (8 ft.). It follows that, unless over 2.5 m (8 ft.) of column height can be reserved for liquid distributor, one must resort to using a distributor with multiple levels of orifices or use more than one single-level orifice distributor, each with a different orifice size. The design features of many of these types of distributors are proprietary.

The pipe-arm distributors depend, for their performance, on the liquid head prevailing upstream of the orifices; this pressure is generated usually by a liquid feed pump. The turndown capability of the pipe-arm distributors are only limited by the capacity of the feed pump and the maximum allowable velocity of liquid through the orifices above which formation of liquid spray might cause entrainment. The biggest drawback of this type of distributor is that the flow variation from orifice to orifice can be excessive, especially at high flow rates due to variability of the size and shape of the orifices and the pressure drop through the pipe arms. Therefore, orifice-pan and orifice-through distributors are generally preferred for both pilot plant distillation columns and industrial distillation columns.

The number of liquid distribution points required for unit tower cross-sectional area is a function of the type and size of the packing. Based on the authors' experience, the following general statements can be made:

  • Large random packings require fewer pour points than smaller random packings.
  • Large structured packings require fewer pour points than medium sized structured packings.
  • Small structured packings have better liquid spreading characteristics than larger structured packings.
  • Except for small random packings, most packings will operate well with pour point densities of between 40 points/m2 (4/ft2) and 60 points/m2 (6/ft2). Even small random packings of commercial interest perform well with 100 points/m2.
  • The smallest size orifice used is 2-3 mm in diameter; this small orifice can only be used with clean systems.
  • Sufficient liquid head should be allowed to limit the individual orifice flow variation to ± 5% of the average flow rate.
  • The layout of liquid distributor orifices over the tower cross-sectional area is based on the method of Moore and Rukovena (1986)

Download a legacy print-ready version of this article with high-resolution versions of Figures 1 and 2

Separation Technology Articles